In the treatment of flue gases from fossil fuel power plants and from garbage incinerators and the like, it is common practice to scrub the flue gas by passing the flue gas from below upwardly through a scrubber having at least one scrubbing zone and in counterflow to an absorption which is supplied to an upper part of the scrubbing zone and which is withdrawn below the scrubbing zone. The absorption liquid can be circulated through a scrubbing liquid circulation path to the upper part of the scrubbing zone and ammoniacal water (aqueous ammonium hydroxide solution) can be supplied to the scrubbing liquid circulation. From this circulation, a portion can be branched and subjected to oxidation in an oxidation unit in which the ammonium sulfite ((NH.sub.4).sub.2 SO.sub.3) and the ammonium hydrogen sulfite (NH.sub.4 HSO.sub.3) is oxidized to ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4).
In the removal of sulfur dioxide from flue gases by scrubbing with aqueous ammonia solutions as the absorbent liquid, the following reactions take place: EQU SO.sub.2 +NH.sub.3 +H.sub.2 O.fwdarw.NH.sub.4 HSO.sub.3 EQU NH.sub.4 HSO.sub.3 +NH.sub.3 .fwdarw.(NH.sub.4).sub.2 SO.sub.3 EQU (NH.sub.4).sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.fwdarw.2NH.sub.4 HSO.sub.3 EQU (NH.sub.4).sub.2 SO.sub.3 +1/2O.sub.2 .fwdarw.(NH.sub.4).sub.2 SO.sub.4
A result of the process is the saleable byproduct ammonium sulfite which has significant value as a fertilizer or an ingredient in fertilizers.
A process of this type is described in DE-C 37 33 319. The intermediate scrubbing zone of a three stage scrubber in this system has a scrubbing liquid circulation in which the absorber is aqueous ammonia solution and to which water is supplied for material balance, e.g. as make up water. From this circulation, a partial stream is branched and passed through the oxidation unit.
In the oxidation unit, the salts dissolved in the solution are oxidized to form the ammonium sulfate which can be recovered by evaporation of the aqueous phase and granulation. The result is an ammonium sulfate granulated product which can be used directly as a fertilizer and is a saleable product.
The circulated absorption liquid can develop a high concentration of dissolved salts ((NH.sub.4).sub.2 SO.sub.3 and NH.sub.4 HSO.sub.3). The absorption liquid in the scrubbing zone has a high NH3 partial pressure which can give rise to aeresols which are detrimental to the health and represent an environmental pollutant.
In the earlier process, a third scrubbing zone is provided downstream of the scrubbing zone utilizing this recirculated ammoniacal solution and in which the scrubbing liquid is acidified by the addition of sulfuric acid, thereby washing out the ammonia to a permissible level thereof. The additional scrubbing zone for the elimination of NH.sub.3 is expensive and in practice it is found that one cannot completely avoid aeresol formation, at least not to the desired degree.
EP-A 0 778 067 describes a process in which ammonium sulfate solution is used as the absorption liquid and in which the ammonium sulfate solution is drawn from a sump of the scrubber which is aerated with ambient air and is then returned at the head of the scrubbing column. The maximum possible oxidation level should be maintained in the scrubbing liquid in the sump. From the scrubbing liquid circulation, sufficient scrubbing solution is discharged to adjust the salt content of the scrubbing liquid to substantially 25 to 40% by weight. The absorption of SO.sub.2 with the ammonium sulfate scrubbing process is, however, unsatisfactory. The scrubbing zone must be excessively long if the SO.sub.2 remaining in the scrubbed flue gas is to be below the permissible tolerances.
Furthermore, the aerosol formation may reach tolerable levels, especially when the smoke which is treated has a high SO.sub.2 concentration say more than 2700 mg/m.sup.3 (STP). Similar processes are known from EP A 0 212 523 and EP A 0 620 187. In these processes as well, an ammonium sulfate solution forms the scrubbing liquid.